1. Field of the Invention
This invention relates to a lens system preferable for use in an image taking lens (image taking optical system) of a video camera, digital camera, or mobile phone and mobile terminal with camera, etc.
2. Description of the Related Art
Various video cameras, digital cameras, and mobile phones and mobile terminals with camera, which are equipped with a CCD sensor, CMOS sensor, or other solid-state image pickup element, have been developed in recent years. Compact, lightweight image taking lenses are strongly desired, especially with mobile telephones and mobile terminals from the standpoint of portability.
There are known compact image taking lenses with a two-lens arrangement, having a first lens with a positive refractive power and a second lens with a negative refractive power (for example, see Patent Documents 1 and 2).
There are also known image taking lenses with a so-called triplet arrangement, in which the improvement of the image forming performance is taken into consideration along with a compact size and having a first lens with a positive refractive power, a second lens with a negative refractive power, and a third lens with a positive refractive power (for example, see Patent Documents 3 to 8).
Among triplet arrangements, there are also known image taking lenses of a so-called front stop arrangement, having an aperture stop positioned at the most object side and being a comparatively advantageous arrangement for reduction of the front lens diameter and elongation of the exit pupil (for example, see Patent Documents 9 to 17).
There are also arrangements, which though not being image taking lenses, employ a triplet arrangement with an aim at realizing a compact size (for example, see Patent Documents 18 and 19).
Also, as front stop type systems which are developed types of the triple arrangement, there are known image taking lenses which are arranged from four lenses (for example, see Patent Documents 20 to 22).
[Patent Document 1]
    Japanese Patent Laid-Open No. 2002-258155 (corresponding to U.S. Pat. No. AA2003081330)[Patent Document 2]    U.S. Pat. No. 5,329,403[Patent Document 3]    Japanese Patent Laid-Open No. 2001-83409 (corresponding to U.S. Pat. No. 6,560,043)[Patent Document 4]    Japanese Patent Laid-Open No. 2002-221659 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 5]    Japanese Patent Laid-Open No. 2002-244030 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 6]    Japanese Patent No. 2,683,463 (corresponding to EP B1 467240)[Patent Document 7]    Japanese Patent No. 2,742,581 (corresponding to U.S. Pat. No. 5,274,456)[Patent Document 8]    U.S. Pat. No. 5,596,455[Patent Document 9]    Japanese Utility Model No. S44(1969)-21105[Patent Document 10]    Japanese Patent Laid-Open No. S53(1978)-36226[Patent Document 11]    Japanese Patent Laid-Open No. S61(1986)-77816[Patent Document 12]    Japanese Patent Laid-Open No. H1(1989)-144007 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 13]    Japanese Patent Laid-Open No. H4(1992)-153612 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 14]    Japanese Patent Laid-Open No. H5(1993)-188284 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 15]    Japanese Patent Laid-Open No. H9(1997)-288235 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 16]    Japanese Patent Laid-Open No. 2001-75006 (corresponding to U.S. Pat. No. 6,466,377)[Patent Document 17]    U.S. Pat. No. 6,441,971[Patent Document 18]    U.S. Pat. No. 4,163,604[Patent Document 19]    U.S. Pat. No. 5,596,452[Patent Document 20]    Japanese Patent Laid-Open No. 2002-228922 (corresponding to U.S. Pat. No. 6,476,982)[Patent Document 21]    Japanese Patent Laid-Open No. 2002-365529 (corresponding to none presently as of Mar. 11, 2004)[Patent Document 22]    Japanese Patent Laid-Open No. 2002-365531 (corresponding to none presently as of Mar. 11, 2004)
With the system disclosed in Japanese Patent Laid-Open No. 2002-258155, a negative lens of a two-lens arrangement has a shape with which a comparatively strongly concave surface is faced towards the image side, the exit pupil distance tends to be short, and the occurrence of shading becomes an issue in the case where a solid-state image pickup element is used.
With the system disclosed in U.S. Pat. No. 5,329,403, the interval between a positive lens and a negative lens of a two-lens arrangement is large, making a compact size difficult to realize.
The lens system of Japanese Patent Laid-Open No. 2001-83409 is of a middle stop type, with which an aperture stop is positioned inside the lens system. Thus in comparison to a front stop type system, a long back focus must be secured in order to set a long exit pupil distance and reduction of the total lens length is difficult. The same can be for the lens systems of Japanese Patent Laid-Open No. 2002-221659 and Japanese Patent Laid-Open No. 2002-244030.
Also in terms of manufacturing cost, it is more effective to use lenses formed of plastic material than lenses formed of glass material. With the lens systems of Japanese Patent No. 2,683,463, U.S. Pat. No. 5,596,455, Japanese Utility Model No. S44(1969)-21105, Japanese Patent Laid-Open No. S53(1978)-36226, Japanese Patent Laid-Open No. S61(1986)-77816, Japanese Patent Laid-Open No. H1(1989)-144007, Japanese Patent Laid-Open No. H4(1992)-153612, and Japanese Patent Laid-Open No. H5(1993)-188284, though the number of component lenses is three and thus few, since all lenses are formed of glass material, the manufacturing cost tends to be high.
With lenses formed of plastic material, shape variations due to temperature and humidity variations tend to occur more readily in comparison to lenses formed of glass material, and focus variation and aberration variations due to environmental variations tend to be a problem. This becomes especially significant when the refractive power of a lens is made high.
With the three-lens arrangements of Japanese Patent No. 2,742,581 and Japanese Patent Laid-Open No. 2001-75006, though all lenses are formed of plastic material to facilitate manufacture, since lenses of strong refractive power are used in part, focus variation and aberration variations due to environmental variations tend to occur.
Though Japanese Patent Laid-Open No. 2002-228922 discloses a lens system using two lenses formed of glass material and two lenses formed of plastic material, the number of lenses is four and thus high, and the reduction of the total lens length is difficult. The same applies to Japanese Patent Laid-Open No. 2002-365529 and Japanese Patent Laid-Open No. 2002-365531.
Also, in the case of a positive-negative-positive, three-lens arrangement, a front stop type arrangement, with which an aperture stop is most separated from an image pickup element, is advantageous for reducing the front lens diameter to achieve a compact size and for providing good telecentric characteristics at the image side. With a front stop type, though a shape which is concentric with respect to the aperture stop is preferable for obtaining good optical performance while achieve reduction of the total length, with the conventional examples, the shapes of the respective lenses were not concentric with respect to the aperture stop or even if they were concentric, they were hardly optimal in terms of shape.